Area Forecast Discussion
Issued by NWS Des Moines, IA

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FXUS63 KDMX 182356

Area Forecast Discussion
National Weather Service Des Moines IA
656 PM CDT Thu May 18 2017

.SHORT TERM.../Tonight through Friday/
Issued at 348 PM CDT Thu May 18 2017

Strong upper level trough of low pressure located over the
central and southern Rockies will continue to pump moisture into
the region through tomorrow. Shortwave embedded within the
southwest flow aloft looks to enter southwest Iowa by around 06z
Friday and move north-northeast across the state through midday
Friday. Large swath of strongest Theta-E advection moves through
the forecast area b/t 09z to 15z. Precipitable water values range
from 1.0 to 1.2 inches tomorrow morning which is roughly 125% of
normal for this time of year in southern Iowa. Warm layer cloud
depths only in the 10kft to 11.5kft range with fairly weak
moisture transport into the state. Not the greatest setup for
efficient rain producers, but a few areas may top an inch by
midday Friday. More of a long duration rainfall event (6-12 hours)
and thus low confidence with any widespread flash flooding attm.
Plus the deep layer moisture moves into northern Iowa by 15-18z
Friday and expecting to see a break in the rain during the
afternoon hours from south to north before the next wave pushes
into the state Friday night. Have the highest QPF going b/t 06z-
12z Friday across the central to southwest portions of the
forecast area. The other issue will be the stout east-northeast
winds increasing overnight and persisting through the morning
hours Friday creating a fairly miserable day tomorrow.

.LONG TERM.../Friday night through Thursday/
Issued at 348 PM CDT Thu May 18 2017

Precip trends will be the primary concern through the period, one
heading into Saturday and another Monday into Tuesday. The former
will be the more significant with moderate to locally heavy rains
possible. This will be driven by the impressive NV/UT PV
anomaly/short wave as it lifts into the Plains and Missouri Valley
Saturday. This will produce deep, widespread lift driven by upper
level divergence/diffluence aloft and mid level DPVA. A nice
baroclinic zone aloft will already be established with
warm/theta-e advection and Fn frontogenetical response increasing
as well. Moisture convergence into the baroclinic zone will be
quite strong per 300-305K isent surfaces so expect moderate to
heavy, efficient rainfall. Warm cloud depths are nudging higher
higher to 11K ft and precipitable water values are now moving
further through the 90th percentiles as well. Instability looks
rather low, but effective shear will be elevated so there could be
some marginal hail in elevated convection south.

There will be decreasing PoPs into the day Saturday with the
approach of a dry slot and surface low tracking through the heart
of Iowa. Synoptic scale concerns will decrease but most models now
bring at least parts of eastern Iowa into the warm sector. There
isn`t really a consensus among the models however. The NAM and
ECMWF are less bullish, with the NAM being too far east and the
ECMWF too fast to realize afternoon insolation. The GFS is a bit
more optimistic however and suggests MLCAPEs approaching 1000 j/kg
with plenty of deep shear. Thus the overall severe potential is
still only marginal at best, but something to watch. Some weak
wrap around precip may linger into the evening but the end of the
weekend should be dry with cool temperatures.

This will usher in a prolonged period of NW flow as the parent low
lifts into the Great Lakes and east. Models are in fairly good
agreement with another short wave dropping from the Dakotas into
the MS Valley. This will result in a period of precip Mon into
Tue. Models are not in good agreement to end the forecast period
with the ECMWF showing much strong ridging into the Plains for
Thursday while the GFS shows a very weak short wave in remaining
NW flow. Forecast has slight chances far NE to account for this,
but will have to watch for future warmer, drier trends.


.AVIATION.../For the 00Z TAFS through 00Z Friday evening/
Issued at 655 PM CDT Thu May 18 2017

Pretty much all downhill from here. Stalled boundary south of
state will enhance lift as first in series of waves arrives
overnight into Friday. Area of thunderstorms over KS/MO will
weaken but provide period of near continuous rain through 18z
...then lowering cigs to IFR and VSBY to MVFR as sfc boundary
nears the area by 00z Friday. East northeast winds will be gusty
from 09-18z and begin to lessen in speed south aft 19z. Little
improvement expected beyond end of period due to second system
arriving Friday night. /rev


Issued at 348 PM CDT Thu May 18 2017

The aforementioned efficient rain mechanisms will likely result
in widespread 1-2 rains by the weekend with 2-3 possible
central and south. This doesn`t take into account the potential
for locally higher amounts. No headlines have been issued as of
yet however as these rains will be drawn out from tonight through
Friday Night and the lack of instability should preclude much
convection and heavier amounts in just a few hours. Nevertheless
Flood Watches for individual river forecast points will likely be
needed either later tonight or Friday as rain begins to accumulate
and the RFC 24hr QPF window floats into Friday Night. RFC
contingency forecasts suggest multiple locations will reach at
least minor flood stage with isolated moderate flooding possible
depending on which basins are affected by the heavier amounts.




SHORT TERM...Podrazik
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